Rocker Bogie Project Report and Ppt

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ABSTRACT

Taking six-wheeled rocker-bogie lunar rover as an object, on the basis of force analysis between the wheels and lunar soil, its obstacle-climbing force model on loose soil was established in this paper, and the wheel sinkages were obtained. Based on the method of solving the wheel’s driving torque solution space feasible regions, this paper analyzed the forward obstacle-climbing capability of the six-wheeled rocker-bogie lunar rover on loose soil, including single-wheel obstacle-climbing and two wheels obstacle-climbing simultaneously. Simulations show that under the loose soil environment, the wheels have the different obstacle-climbing capability, i.e. the rear wheel is the best, the middle one is the worst and the front one is moderate.

It is obvious that rovers are important vehicles of today’s solar system exploration. Most of the cover designs have been developed for Mars and Moon surface in order to understand the geological history of the soil and rocks. Exploration operations need high speed and long distance traversal in a short mission period due to environmental effects, climate and communication restrictions. Several mechanisms have been suggested in recent years for suspensions of rovers on rough terrain. Although their different mechanisms have found widespread usage in mobile robotics, their low operation speed is still a challenging problem. In this research, a new suspension mechanism has been designed and its kinematic analysis results were discussed. Standard rocker-bogie suspension mechanism, which has been developed in the late 1990s, has excellent weight distribution for different positions on rough terrain. New design, mostly similar to the rocker-bogie suspension system, has a natural advantage with linear bogie motion which protects the whole system from getting rollover during high-speed operations. This improvement increases the reliability of structure on field operations and also enables the higher speed exploration with same obstacle height capacity as rocker-bogie.

In this thesis study, new bogie mechanism consisted of double-lambda mechanisms, which has been firstly presented by Pafnuty Lvovich Chebyshev in 1869, is solved by analytically to define the positions and singular configurations. A new structural synthesis formula also has been introduced for such suspension mechanisms with lower and higher kinematic pairs. By using structural synthesis methods, a suspension mechanism has been designed with double-lambda mechanism. Equivalent force and moment functions were also derived from the equation of motion method. The results are confirmed with the computer analysis made by Visual Nastran 4D. For this purpose, a computer model has been constructed and assembled with the same design parameters of NASA Mars Exploration Rovers (MER1 and MER2).  

INTRODUCTION

The Rocker-Bogie system is the suspension arrangement used in the Mars rovers (mechanical robot) introduced for the Mars Pathfinder and also used on the Mars Exploration Rover (MER) and Mars Science Laboratory (MSL) missions. It is currently NASA‘s favored design. The term “rocker” comes from the rocking aspect of the larger links on each side of the suspension system.

These rockers are connected to each other and the vehicle chassis through a differential. Relative to the chassis, when one rocker goes up, the other goes down. The chassis maintains the average pitch angle of both rockers. One end of a rocker is fitted with a drive wheel and the other end is pivoted to a bogie.

The term “bogie” refers to the links that have a drive wheel at each end. Bogies were commonly used as load wheels in the tracks of army tanks as idlers distributing the load over the terrain. Bogies were also quite commonly used on the trailers of semi-trailer trucks. Both applications now prefer trailing arm suspensions.

WORKING OF ROCKER BOGIE

The Rocker Bogie design has no springs and stub axles for each wheel, allowing the rover to climb over obstacles, such as rocks, that are up to twice the wheel’s diameter in size while keeping all six wheels on the ground. As with any suspension system, the tilt stability is limited by the height of the center of gravity.

Systems using springs tend to tip more easily as the loaded side yields. Based on the center of mass, the Curiosity rover of the Mars Science Laboratory mission can withstand a tilt of at least 50 degrees in any direction without overturning, but automatic sensors limit the rover from exceeding 30-degree tilts. The system is designed to be used at a slow speed of around 10 cm/s, so as to minimize dynamic shocks and consequential damage to the vehicle when surmounting sizable obstacles. Each of the rover’s six wheels has an independent motor. The two front and two rear wheels have individual steering motors which allow the vehicle to turn in place. Each wheel also has cleats, providing grip for climbing in soft sand and scrambling over rocks.

The maximum speed of the robots operated in this way is limited to eliminate as many dynamic effects as possible so that the motors can be geared down, thus enabling each wheel to individually lift a large portion of the entire vehicle’s mass. In order to go over a vertical obstacle face, the front wheels are forced against the obstacle by the center and rear wheels. The rotation of the front wheel then lifts the front of the vehicle up and over the obstacle. The middle wheel is then pressed against the obstacle by the rear wheels and pulled against the obstacle by the front until it is lifted up and over.

Finally, the rear wheel is pulled over the obstacle by the front two wheels. During each wheel’s traversal of the obstacle, forward progress of the vehicle is slowed or completely halted. This is not an issue for the operational speeds at which these vehicles have been operated to date.  

ADVANTAGES OF ROCKER BOGIE

1. Used in various types of road or various climbing roads.
2. Useful system.
3. Not many parts are used.

DISADVANTAGES OF ROCKER BOGIE

1. Number of wires used.

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